It has been known for a long time that by cooling and condensing sulfuric acid vapours in air and in air containing aqueous vapour (steam) a sulfuric acid mist is formed, i.e. an aerosol of minute droplets of sulfuric acid. In U.S. patent specification No. 2,017,676 it has been proposed to counteract the formation of the acid mist by cooling a gas containing SO.sub.3,H.sub.2 SO.sub.4 vapour and H.sub.2 O in vertical, narrow ceramic tubes surrounded by a layer of sand the purpose of which is to delay the cooling of the gas, and by an outer metal tube; coolant, preferably water, is in contact with the outer surface of the metal tubes. In this manner there can only be obtained sulfuric acid of low concentration and the gas discharged from the top of the tubes contains more acid mist than allowed under present environmental demands.
In DK patent specification No. 145,457 (corresponding to U.S. Pat. No. 4,348,373) there is disclosed a process for preparing concentrated sulfuric acid from gases containing SO.sub.3 and excess of H.sub.2 O. The gas is cooled and the sulfuric acid condensed and concentrated in two steps in an absorption tower containing filler bodies. In the lowermost step the feed gas is passed upwardly countercurrently with the condensed acid, the concentration of which is thereby increased. In the subsequent step the sulfuric acid vapour is absorbed in sulfuric acid recycled through the layer containing filler bodies. The content of sulfuric acid mist is kept down by virtue of a specified regulation of the temperature at which the recycle acid is removed from the tower. According to this patent specification remaining acid mist is removed in an aerosol filter placed after the absorption zone. The filter is a "low velocity" filter operating at a linear velocity below 1 m/s and with a pressure drop above 20-30 mbar.
In DK patent application No. 1361/82 (corresponding to GB patent specification No. 2,117,368) a process is disclosed for preparing sulfuric acid in a sulfuric acid tower described in the specification. The tower is constructed as a tubular heat exchanger having two horizontal tube sheets and a bundle of vertical, acid resistant tubes extending into an inlet compartment below the lower tube sheet.